Why is this an issue?

The nullish coalescing operator ?? allows providing a default value when dealing with null or undefined. It only coalesces when the original value is null or undefined. Therefore, it is safer and shorter than relying upon chaining logical || expressions or testing against null or undefined explicitly.

This rule reports when disjunctions (||) and conditionals (?) can be safely replaced with coalescing (??).

The TSConfig needs to set strictNullChecks to true for the rule to work properly.

How to fix it

Rewrite the logical expression || using ?? on the unchecked operands.

Code examples

Noncompliant code example

function either(x: number | undefined, y: number) {
  return x || y;
}

Compliant solution

function either(x: number | undefined, y: number) {
  return x ?? y;
}

Noncompliant code example

function either(x: number | undefined, y: number) {
  return x !== undefined ? x : y;
}

Compliant solution

function either(x: number | undefined, y: number) {
  return x ?? y;
}

Resources

• MDN - Nullish coalescing